Chapter 1

Elements of FORTRAN

Standards Conformance

f77 was designed to be compatible with the ANSI X3.9-1978 FORTRAN standard and the corresponding International Organization for Standardization (ISO) 1539-1980, as well as standards FIPS 69-1, BS 6832, and MIL-STD-1753. (This is no longer the current Fortran standard.)

Floating-point arithmetic for both compilers is based on IEEE standard 754-1985, and international standard IEC 60559:1989.

On SPARC platforms, both compilers provide support for the optimization-exploiting features of SPARC V8, and SPARC V9, including the UltraSPARCTM implementation. These features are defined in the SPARC Architecture Manuals, Version 8 (ISBN 0-13-825001-4), and Version 9 (ISBN 0-13-099227-5), published by Prentice-Hall for SPARC International.

In this document, "Standard" means conforming to the versions of the standards listed above. "Non-standard" or "Extension" refers to features that go beyond these versions of these standards.

The responsible standards bodies may revise these standards from time to time. The versions of the applicable standards to which these compilers conform may be revised or replaced, resulting in features in future releases of the Fortran compilers that create incompatibilities with earlier releases.

Radix point, delimiter for logical constants and operators, record fields

'

Apostrophe

Quoted character literals

"

Quote

Quoted character literals, octal constants

$

Dollar sign

Delimit namelist input, edit descriptor, directives

!

Exclamation

Comments

:

Colon

Array declarators, substrings, edit descriptor

%

Percent

Special functions: %REF, %VAL, %LOC

&

Ampersand

Continuation, alternate return, delimit namelist input; use in column 1 establishes the line as a tab-format source line

?

Question mark

Request names in namelist group

\

Backslash

Escape character

< >

Angle brackets

Enclose variable expressions in formats

Note the following usage and restrictions:

Uppercase or lowercase is not significant in the key words of FORTRAN statements or in symbolic names. The -U option of f77 makes case significant in symbolic names.

Most control characters are allowed as data even though they are not in the character set. The exceptions are: Control A, Control B, Control C, which are not allowed as data. These characters can be entered into the program in other ways, such as with the char() function.

Any ASCII character is valid as literal data in a character string.

For the backslash (\) character, you may need to use an escape sequence or use the -xl compiler option. For the newline (\n) character, you must use an escape sequence. See also TABLE 2-3.

Symbolic names can be any number of characters long . The standard is 6.

Symbolic names consist of letters, digits, the dollar sign ($), and the underscore character (_). $ and _ are not standard.

Symbolic names generally start with a letter--never with a digit or dollar sign ($). Names that start with an underscore (_) are allowed, but may conflict with names in the Fortran and system libraries.

Note  Procedure names that begin with exactly two underscores are considered special support functions internal to the compiler. Avoid naming functions or subroutines with exactly two initial underscores (for example __xfunc) as this will conflict with the compiler's usage.

Uppercase and lowercase are not significant; the compiler converts them all to lowercase. The -U option on the f77 command line overrides this default, thereby preserving any uppercase used in your source file.

Example: These names are equivalent with the default in effect:

ATAD = 1.0E-6

Atad = 1.0e-6

The space character is not significant.

Example: These names are equivalent:

IF ( X .LT. ATAD ) GO TO 9

IF ( X .LT. A TAD ) GO TO 9

IF(X.LT.ATAD)GOTO9

Here are some sample symbolic names:

TABLE 1-3 Sample Symbolic Names

Valid

Invalid

Reason

X2

2X

Starts with a digit.

DELTA_TEMP

__DELTA_TEMP

Starts with an __ (reserved for the compiler).

Y$Dot

Y|Dot

There is an invalid character |.

In general, for any single program unit, different entities cannot have the same symbolic name. The exceptions are:

A variable or array can have the same name as a common block.

A field of a record can have the same name as a structure.

A field of a record can have the same name as a field at a different level of the structure.

Throughout any program of more than one programming unit, no two of the following can have the same name:

Block data subprograms

Common blocks

Entry points

Function subprograms

Main program

Subroutines

Program Units

A program unit is a sequence of statements, terminated by an END statement. Every program unit is either a main program or a subprogram. If a program is to be executable, it must have a main program.

There are three types of subprograms: subroutines, functions, and block data subprograms. The subroutines and functions are called procedures, which are invoked from other procedures or from the main program. The block data subprograms are handled by the loader.

Statements

A statement consists of one or more key words, symbolic names, literal constants, and operators, with appropriate punctuation. In FORTRAN, no keywords are reserved in all contexts. Most statements begin with a keyword; the exceptions are the statement function and assignment statements.

Executable and Nonexecutable Statements

Every statement is either executable or nonexecutable. In general, if a statement specifies an action to be taken at runtime, it is executable. Otherwise, it is nonexecutable.

Padding

Comments and Blank Lines

A line with a c, C, *, d, D, or! in column one is a comment line, except that if the -xld option is set, then the lines starting with D or d are compiled as debug lines. The d, D, and! are nonstandard.

If you put an exclamation mark (!) in any column of the statement field, except within character literals, then everything after the ! on that line is a comment.

Directives

A directive passes information to a compiler in a special form of comment. Directives are also called compiler pragmas. There are two kinds of directives:

General directives

Parallel directives

See the Sun WorkShop Fortran User's Guide and the Fortran Programming Guide for details on the specific directives available with f77.

General Directives

The form of a general directive is one of the following:

C$PRAGMA id

C$PRAGMA id ( a [ , a ] ... ) [ , id ( a [ , a ] ... ) ] ,...

C$PRAGMA SUN id[=options]

The variable id identifies the directive keyword; a is an argument.

Syntax

A directive has the following syntax:

In column one, any of the comment-indicator characters c, C, !, or *

In any column, the ! comment-indicator character

The next 7 characters are $PRAGMA, no blanks, any uppercase or lowercase

Rules and Restrictions

After the first eight characters, blanks are ignored, and uppercase and lowercase are equivalent, as in FORTRAN text.

Because it is a comment, a directive cannot be continued, but you can have many C$PRAGMA lines, one after the other, as needed.

If a comment satisfies the above syntax, it is expected to contain one or more directives recognized by the compiler; if it does not, a warning is issued.

Parallelization Directives

Parallelization directives explicitly request the compiler attempt to parallelize the DO loop that follows the directive. The syntax differs from general directives. Parallelization directives are only recognized when compilation options -parallel or -explicitpar are used. (f77 parallelization options are described in the Fortran Programming Guide.)

Parallelization directives have the following syntax:

The first character must be in column one.

The first character can be any one of c, C, *, or !.

The next four characters are $PAR, no blanks, either upper or lower case.

Next follows the directive keyword and options, separated by blanks.

The explicit parallelization directive keywords are:

TASKCOMMON, DOALL, DOSERIAL, and DOSERIAL*

Each parallelization directive has its own set of optional qualifiers that follow the keyword.

Example: Specifying a loop with a shared variable:

C$PAR DOALL SHARED(yvalue)

See the Fortran Programming Guide for details about parallelization and these directives. Fortran parallelization features require a Sun WorkShop HPC license.